THE COMMON MARINE SHELLS OF THE BOMBAY SHORE, 337 
the ventral border is the edge of the shell opposite the umbo and which 
- constitutes the length of the shell. The breadth is, of course, a measure- 
ment from the umbo to the ventral border in a straight line, and this 
measurement also determines whether a valve is equilateral or not, 2.e., 
whether the umbo is centrally placed with regard to the shape of the 
valve. When dimensions are given, they are of what I believe to be 
full-sized specimens. 
Unfortunately nearly all the common bivalves belong to Division I, 
but two large subdivisions are fairly easily determined—the Integri- 
pallia and the Stnupallia. Still regarding the inside of the shell, there 
will be found running parallel to the ventral border a line or scar 
formed by the muscular attachment of what is termed the mantle 
of the animal to the shell. In cases where this line (the pallial line) 
simply follows the outline of the shell between the two muscular scars, 
it indicates that the animal has no siphons and belongs to the class 
Integripallia. Where this line is inflected and forms a sinus or bay, 
the animal has siphons—the Srnupallia. 
Division I. 
INTEGRIPALLIA.—Licgament Externat. 
ARCACHA. 
There are many species of this family, and it is easily recognised 
by the long straight row of similar and comb-like teeth of the hinge. 
Shells generally white and covered more or less with a rough 
or bristly epidermis, also radiately ribbed from the umbo. 
Arca inequivalvis (Brug.) is a large thick swollen shell with about 
34 broad smooth ribs. Umboes curled over the broad ligamental area, 
Length 24—38", breadth 2—22", depth of double shell 2”. 
Arca bistrigata (Dunker) lives attached to rocks or stones by a 
horny byssus coming out of the centre of the ventral border, and can 
only be removed by the use of a considerable amount of force. It is 
generally a rough dirty-looking shell often covered with limy matter 
and a bristly epidermis. The shell is oblong, and the hinge line is very 
long and straight. 
Ribs numerous, slightly noduled and in pairs at the sides. 
Size 2—22" X12". 
